Fructose and the Brain

New brain imaging studies show how fructose affects the appetite – and our likelihood to overeat.

There’s no question that fructose, particularly high fructose corn syrup, has been under the gun in recent years. Its prevalence as a food sweetener has led experts to explore its health effects, and much of the research has found that it actually affects the body differently from other sugars, like glucose, because of the ways in which the sugars are metabolized. A new study suggests that fructose may also have a specific affect on the brain areas and hormones that control hunger and whether we feel satiated — full — or not. This could, in turn, have an affect on our tendency to overeat.

Glucose increased the connection between these areas and how satisfied and full the participants said they felt after drinking the beverage. Fructose, on the other hand, was not associated with these changes.

The researchers studied the effects of fructose and glucose on the brains of 20 participants who consumed drinks containing one of the two sugars. The team was specifically interested in cerebral blood flow (CBF), which is a good gauge of the activity level of specific brain regions.

They used an MRI to look at the hypothalamus (an area that governs basic bodily functions, like temperature, feelings of stress, thirst, appetite, and hunger) as well as the motivation and reward centers in the brain. What they found was that glucose consumption was linked to lower CBF in these areas.

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Glucose increased the connection between these areas and how satisfied and full the participants said they felt after drinking the beverage. Fructose, on the other hand, was not associated with these changes, but was generally linked to reduced levels of insulin, the hormone responsible for bringing blood sugar into cells and involved in our experience of satiety after a meal.

"Increases in fructose consumption have paralleled the increasing prevalence of obesity,” the authors point out, “and high-fructose diets are thought to promote weight gain and insulin resistance.” Other studies in non-human animals have found that fructose provokes feeding, while glucose promotes feelings of being full. In other words, because fructose doesn’t trigger the same cascade of events leading us to feel satisfied after eating, we might continue our hunt for food (and open another bag of potato chips) in an effort to arrive at that feeling of satiation.

Cutting fructose out of the diet completely is probably not the answer. Foods that naturally contain fructose (like fruits) offer important nutrients, and the amount of fructose they contain is not nearly as high as that of foods sweetened with HFCS. But cutting down on processed foods and drinks sweetened with high-fructose corn syrup (HFCS) is probably a good idea, the researchers say.